Rice 'diabetes risk' overstated

New research has found that “eating white rice could raise your risk of type 2 diabetes” according to the Daily Mail.

The starch-rich staple can potentially release high amounts of sugar into the blood when digested, and so researchers have previously speculated that it may contribute towards type 2 diabetes, a condition where the body finds it difficult to regulate blood sugar.

In this new research, scientists examined the issue by pulling together data from four previous studies that had examined over 350,000 participants. Across the four studies 4% of participants developed diabetes, and greater rice intake was associated with higher risk of developing type 2 diabetes. This link was evident when researchers separately analysed two of the studies conducted in Asian participants who ate four portions of white rice a day on average, but no such link was found in the two studies in Western populations.

Although the review has found an association, it cannot prove that white rice itself directly causes type 2 diabetes, as there are many other factors that could affect the risk of developing the condition (such as physical activity, alcohol and obesity). The four individual studies varied in the factors they accounted for.

Eating a healthy balanced diet and taking regular exercise are the best ways of reducing type 2 diabetes risk. Where white rice is concerned, perhaps this study is best considered as supporting the idea of ‘everything in moderation’.

Where did the story come from?

The study was carried out by researchers from Harvard School of Public Health, Harvard Medical School and the Brigham and Women’s Hospital. It was funded by US National Heart, Lung, and Blood Institute and published in the peer-reviewed British Medical Journal (BMJ).

The news generally reflected the research findings but did not make it apparent that the research looked at both Asian and Western populations or that it did not find consistent results across the two groups.

What kind of research was this?

This was a systematic review and analysis of previous studies on the relationship between rice intake and the development of diabetes. It only used the results from prospective studies, meaning those that looked at rice intake in people without diabetes and then followed them over time to see if they developed the condition.

This confirms that the participants’ rice intake preceded their development of diabetes. The review featured a statistical pooling of results (meta-analysis) that allowed the researchers to analyse the subjects of all previous studies as a single group.

A systematic review gathers together all relevant high-quality studies on a subject. It is the best way to determine what existing evidence indicates about a particular question.

When combining such studies in a systematic review, the authors need to take into consideration differences between the underlying studies when interpreting their findings and deciding whether to pool the studies. In this case, for example, the studies may have:

used different methods for assessing dietary intake

included different populations

followed participants for a variable length of time

examined outcomes differently

The ideal way to look at the effect of rice intake on diabetes risk would be conducting randomised controlled trials. However, this would be unlikely to be feasible, particularly in this case as people would have to stick with the allocated diet for long periods in order to look at an outcome such as diabetes development. Therefore, prospective cohort studies are the best way of looking at whether this type of exposure (in this case rice intake) is associated with a particular outcome (in this case, development of diabetes).

The main limitation with cohort studies such as the ones pooled in this review is that they may not have adjusted for all relevant factors that could be associated with intake of rice and with risk of diabetes. These include other dietary factors such as alcohol intake, physical activity and being overweight or obese. Also, studies assessing food intake can be particularly prone to some inaccuracy. Participants usually have to estimate their typical dietary intake, which can be hard to recall and variable over time.

What did the research involve?

The researchers looked at Medline and Embase electronic databases to identify all prospective cohort studies that related to rice intake and type 2 diabetes. They excluded studies where participants reported they had diabetes at the start of the study.

For studies that reported rice intake as servings a week or day, the researchers converted this to grams a day by assuming that each serving was equivalent to 158g of cooked rice. To convert raw rice intake to cooked rice intake they multiplied raw intake levels by 2.5, to account for the typical increase in weight while cooking. The researchers used standard methods to pool the results from the studies and calculated risk of development of type 2 diabetes in relation to an individual’s rice intake.

What were the basic results?

Four prospective cohort studies met inclusion criteria. They included a total of 352,384 participants, all of whom reported being free of diabetes at the start of the study.

Two studies were performed in Asian populations (in China and Japan) and the other two studies in Western populations (the US and Australia). Follow-up in these studies ranged between 4 and 22 years. All studies had assessed dietary intake using food frequency questionnaires. Average rice intake levels varied considerably across studies. For example, the average intake in the Chinese study was four servings (625g) of cooked rice a day, compared to the US study where 98% of participants consumed less than five servings a week.

Studies varied in the potential confounders that they took into account, such as other dietary intake, weight, alcohol and physical activity.

Of the 352,384 participants, 13,284 developed type 2 diabetes, or 4% of the group. Overall, the pooled results from all studies found that:

there was significant heterogeneity when pooling these four studies, which meant that their individual results varied significantly

When the researchers separately analysed the two Asian studies and the two Western studies, they found that Asian subjects with a high rice intake had a 55% greater risk of developing diabetes compared to Asian subjects with a low intake (relative risk [RR] 1.55, 95% CI 1.20 to 2.01). However, there was no significant association between rice intake and diabetes risk in Western people (RR 1.12, 95% CI 0.94 to 1.33).

How did the researchers interpret the results?

Conclusion

Rice is a hugely important staple food for many nations around the world, and also many communities in the UK. However, rice, and particularly white rice, can tend to have a high starch content and therefore may cause rapid increases in blood sugar. Given this property for raising blood sugar, the authors of this review investigated how rice intake might be associated with a person’s risk of developing type 2 diabetes, a chronic condition where the body has difficulty regulating their blood sugar.

To investigate the issue this research has examined the association between white rice consumption and the risk of developing diabetes in both Asian and Western populations. Though overall across the four studies there was an increased risk of diabetes with higher white rice consumption, notably the results differed between studies and were not consistent across the two study populations, with no association between rice intake and diabetes in Western populations.

Separate analyses of the two Asian studies found that those who consumed the highest white rice intake had increased risk of developing diabetes than those who consumed the lowest. As the researchers say, rice is consumed heavily in Asia but relatively infrequently in the West, and the substantial difference in regional rice intake levels may contribute to the inconsistency of results from existing studies.

It is also important to note that there was a low rate of development of diabetes across the studies: only 4% of the entire studied population developed diabetes. The 55% increased risk for Asian populations is a ‘relative risk increase’ in people who had high rice consumption compared to those with low rice consumption, reflecting how the risk varied between the two groups rather than suggesting that 55 out of 100 people got diabetes.

The review does not provide absolute figures of the percentage of people with high intake who developed diabetes and the percentage of people with low intake. For example, if Asian people with the lowest rice consumption had a risk of developing diabetes of 3%, these results would suggest that the risk in the highest consumption group was about 4.7% (approximately a 55% increase on 3%).

Furthermore, it is not possible to conclude from this study that white rice consumption itself directly causes type 2 diabetes. There are many other potential confounders that could affect risk of someone getting diabetes, and the four studies varied in the factors they adjusted for (for example, age, sex, family history of diabetes, other dietary factors, physical activity, alcohol, and being overweight or obese).

Other limitations to this study include the inherent inaccuracies in recall that are often involved when people estimate their dietary intake on a food frequency questionnaire.

Lastly, there should be no conclusion from this study that it is better to eat brown rice than white, or that it is better to eat non-rice carbohydrates; this has not been studied.

Eating a healthy balanced diet and taking regular exercise are the best ways of reducing diabetes risk. Where white rice is concerned, perhaps this study best supports the idea of ‘everything in moderation’.